The present invention relates to an air flow meter for measuring the intake air flow rate to an internal combustion engine, especially, a heating resistor type air flow rate measuring apparatus for measuring the air flow rate in the condition accompanied with a backward flow in a pulsating flow.
The air flow in an internal combustion engine is pulsated by a continuous make and break operation of the intake air valve. The pulsation is so amplified by the effect of the columnar vibration in the intake air duct, and the air flow inside the intake air pipe becomes a backward flow in specific conditions related to the number of rotations of the engine and the aperture of the throttle valve. This backward flow brings about various bad effects in the heating resistor type air flow rate measuring apparatus. As for an apparatus for solving this problem, a prior art apparatus disclosed in Japanese Patent Application Laid-Open No. 1-206223 (1989) is known as an air passage structure having a sub air duct shaped in a letter I (or L) which is used as a means for increasing the accuracy of the measurement of the heating resistor type air flow rate measuring apparatus operated under a condition where a backward flow occurs with a pulsating flow. In this prior art device, the air passage is so configured that the backward flow directly blows against the heating resistor by forming a wall facing against the backward flow.
As for another apparatus for reducing the bad effect of the backward flow, a prior art device is disclosed in Japanese Patent Application Laid-Open No. 62-812 (1987). In this prior art device, similarly to the present invention, by detecting the direction of the air flow using the thermal interference between a couple of heating resistors, the output voltage signals from the heating resistors are altered by judging the direction of the air flow; when the air flow is a forward flow, the output voltage to be used is selected from the output voltage signal of the heating resistor for the forward flow; when the air flow is a backward flow, the output voltage signal from the heating resistor for the backward flow is selected.
In general, it is difficult to measure the direction of the air flow, forward or backward, selectively only by using a single heating resistor. In order to solve this problem, for example, as shown in FIG. 10, in observing the average output of the heating resistor type air flow rate measuring apparatus by varying the boost pressure by making the throttle valve gradually open while the number of rotations of the engine is maintained to be constant, the average output of the air flow rate increases linearly as the intake negative pressure increases under a certain threshold value, and for the boost pressure above a certain threshold value, the average output of the air flow rate is estimated to be larger than the actual air flow rate (which is designated as an over-shooting phenomena). Though the pulsation in the air flow rate in the heating resistor type air flow rate measuring apparatus is relatively small when the throttle valve opens with a small aperture, the amplitude of the pulsation in the air flow rate increases as the throttle valve gets to open, and finally, at a throttle valve angle larger than a certain angle (about 30 to 50°) (in the right region of A in FIG. 10), the pulsation amplitude contains the backward flow component. Thus, when the backward flow occurs, as the heating resistor can not discriminate the direction of the air flow whether forward or backward as described above, the average output of the air flow rate is estimated with the forward flow component as well as the backward flow component, and thus takes on larger values.
By means of forming a wall against the direction of the backward flow as described above for the prior art, and making he air passage structure so that the backward flow may not low directly against the heating resistor, it is possible to reduce the estimation error for the average output. However, the reduced error with this means is only half of the overall error. This is because the amount of forward flow increases as the amount of backward flow increases. Thus, in order to reduce the estimation error due to the backward flow, it is necessary to reduce the output value of the forward flow when the backward flow occurs or subtract the backward flow component from the forward flow component as well as the measurement of the forward flow component. There is a prior art apparatus related to this solution in which, in case the backward flow is observed by detecting the direction of the air flow by comparing the output signals from those two heating resistors using a couple of heating resistors as disclosed in the other prior art described above, the backward flow component is subtracted from the forward flow component. This method has yet another problem. One is related to the reduction of resolution in supplying data to the micro computer. DC voltage handled by many micro computers used for general automotive applications is between 0 and 5.12 (V). However, in this method where both the forward flow and the backward flow have a similar relationship between the air flow rate and the output voltage, the resolution of the forward flow is reduced. In an extreme case, the threshold voltage 2.56(V) is used and the range below 2.56(V) is for the range of the output voltage for the backward flow and the range over 2.56(V) is for the range of the output voltage for the forward flow. Thus, the resolution of the output voltage for the forward flow in this case is half of the resolution when the entire range between 0 and 2.56(V) can be used for the forward flow. Though the threshold voltage 2.56(V) in this case is a somewhat extreme case, the resolution for the forward flow is reduced because the threshold voltage should be determined between 1(V) and 2(V) in order to measure the backward flow precisely.
If the heating resistor has a thermal response delay, the detection of the backward flow is delayed when comparing the output signals from the heating resistors. This detection delay has an effect on the measurement precision. This can be illustrated with FIGS. 11A and 11B; when the backward flow begins to rise up at point B in FIG. 11A, the output signal level of the backward flow does not exceed the output signal level of the forward flow, and therefore, the existence of the backward flow is not proved until the output signal level of the backward flow reaches point C; thus, the detection of the backward flow is so delayed.
Further, as disclosed in Japanese Patent Application Laid-Open No. 62-812 (1987), the conventional apparatus determines a direction of air flow by using two heating resistors and produces an output signal by using either one of the detection signals. A noise component produced due to the mutual interference between the two heating resistors and included in the output signal is moderated by attenuating the alternating current component.
However, because the output signal is attenuated in the prior art apparatus, there is the problem that the delay of detection becomes large at the time when the air flow is inverted and thus the precision of the measurement deteriorates.